1. Field of the Invention
The present invention relates to a method for manufacturing a single crystal silicon solar cell and a single crystal silicon solar cell.
2. Description of the Related Art
Solar batteries mainly containing silicon are classified a single crystal silicon solar cell, a polycrystalline silicon solar cell, and an amorphous silicon solar cell, depending on crystallinity. Among those, the single crystal silicon solar cell is prepared as follows: single crystal ingot based on crystal pulling is sliced into a wafer with a wire saw, the wafer is processed into a thickness of 100 to 200 μm, and a pn junction, an electrode, a protective film, and the like are formed thereon to complete a solar cell.
As for polycrystalline silicon, molten metal silicon is crystallized through molding, not crystal pulling, to manufacture polycrystalline ingot. The ingot is sliced into a wafer with a wire saw similar to the single crystal silicon solar cell. The wafer is similarly processed into the thickness of 100 to 200 μm. A pn junction, an electrode, and a protective film are formed similar to the cell single crystal silicon substrate to complete a solar cell.
As for amorphous silicon solar cell, for example, a silane gas is decomposed in a gas phase through discharging by a plasma CVD method to form an amorphous silicon hydride film on the substrate, and diborane, phosphine, and the like are applied as a doping gas and deposited at the same time to concurrently perform a step of forming a pn junction and a film formation step, and an electrode and a protective film are formed to complete a solar cell. In the amorphous silicon solar cell, amorphous silicon absorbs incident light as a direct transition type, so its optical absorption coefficient is about one digit higher than that of the single crystal and polycrystalline silicon (“Solar photovoltaic power generation”, p. 233, by Kiyoshi Takahashi, Yoshihiro Hamakawa, and Akio Ushirokawa, Morikita Shuppan, 1980). Thus, the amorphous silicon has an advantage that a requisite thickness of the amorphous silicon layer is only about 1 μm that is about 1/100 of that of the crystalline solar cell. In consideration of the fact that the production of solar batteries per year exceeds 1 gigawatt in recent years, and will be increased in future, a thin-film amorphous silicon solar cell that enables efficient utilization of resources is very promising.
However, a rate of utilization of the resources cannot be determined simply through comparison with a film thickness necessary for a crystalline solar cell if considering that high-purity gas materials such as silane or diborane are used for manufacturing an amorphous silicon solar cell and that a rate of utilization of the gas materials includes that of the material deposited on portions other than the substrate in a plasma CVD device. In addition, a conversion efficiency of the crystalline solar cell is about 15%, while that of the amorphous silicon solar cell is about 10%. In addition, a problem about degradation in output characteristics due to irradiation with light remains to be solved.
To that end, many attempts to develop a thin-film solar cell with a crystalline silicon material have been made (“Solar photovoltaic power generation”, p. 217, by Kiyoshi Takahashi, Yoshihiro Hamakawa, and Akio Ushirokawa, Morikita Shuppan, 1980). For example, a polycrystalline thin film is formed on an alumina substrate or graphite substrate with a trichlorosilane gas or a tetrachlorosilane gas. The thus-formed film involves many crystal defects, and its conversion efficiency is low. To improve the conversion efficiency, zone melting should be performed to improve crystallinity (see Japanese Unexamined Patent Application Publication No. 2004-342909, for instance). However, even though the zone melting is carried out, there remains a problem of lowering photocurrent response characteristics within a long wavelength band due to reduction in lifetime and a leak current in the crystal grain boundary.